Process for the selective hydrogenation of multiply unsaturated hydrocarbons in olefin-containing hydrocarbon mixtures

ABSTRACT

The present invention describes a process for the parallel selective hydrogenation of branched and unbranched multiply unsaturated C 4 -C 6 -hydrocarbons in olefin-containing hydrocarbon mixtures with minimization of hydrogenation and isomerization of the olefins present in the stream.

The present invention describes a process for the parallel selectivehydrogenation of branched and unbranched multiply unsaturatedC₄-C₆-hydrocarbons in olefin-containing hydrocarbon mixtures withminimization of hydrogenation and isomerization of the olefins presentin the stream.

Industrial C₄-C₆-hydrocarbon mixtures from catalytic crackers or steamcrackers usually contain not only saturated and monounsaturatedcompounds but also multiply unsaturated compounds. Before individualcompounds can be isolated from these mixtures, it is frequentlynecessary to remove other compounds as completely as possible. This canbe effected by physical methods such as distillation, extractivedistillation or extraction and also by means of a selective chemicalreaction of the components to be removed.

C₄ streams from steam crackers or catalytic crackers can, for example,have the following composition:

Component Proportion by mass/% 1,3-Butadiene 42 Isobutene 25 1-Butene 16cis-2-Butene 4 trans-2-Butene 5 n-Butane 5 Isobutane 2 Acetylenes 0.9C5+ 0.1

The work-up of this mixture can, in one variant, be carried out byfirstly reducing the concentration of 1,3-butadiene by means ofextractive distillation or by means of a selective hydrogenation processfor high butadiene concentrations to a value of about 1%. At the sametime, the acetylenes present in the mixture are removed or convertedinto monounsaturated/saturated compounds. The C₄ mixture is referred toas raffinate 1 by those skilled in the art. In the next step, isobuteneis removed, e.g. by converting it by means of methanol into methyltert-butyl ether (MTBE) and removing the latter by distillation. If pureisobutene is to be isolated, the methyl tert-butyl ether cansubsequently be cleaved to form isobutene and methanol again.

For the further work-up of the C₄ mixture, now raffinate 2, the multiplyunsaturated compounds still remaining have to be converted by means of aselective hydrogenation process into the corresponding monounsaturatedand saturated compounds. Now, 1-butene and isobutene can be separatedoff in sufficient purity by distillation and the remaining 2-butenes andthe n-butane can be worked up further. The 2-butenes are frequentlyconverted by dimerization into octenes which are subsequently convertedby means of hydroformylation into PVC plasticizer alcohols. Thesaturated C₄-hydrocarbons can, for example, be used as blowing agents.

If the concentration of the multiply unsaturated compound is not reducedto a value of less than 10 ppm before the 1-butene is separated off inthe selective hydrogenation process, the purity requirements for1-butene used in polymerizations are not met. Furthermore, multiplyunsaturated compounds reduce the catalytic activity of these catalystsfor the dimerization of the 2-butenes.

The work-up of C₄ streams from steam crackers or catalytic crackers isdescribed in principle in K.-D. Wiese, F. Nierlich, DGMK-Tagungsbericht2004-3, ISBN 3-936418-23-3.

The selectivities in the processes for selective hydrogenation ofmultiply unsaturated hydrocarbons have to meet particularly demandingrequirements since products of value are destroyed in the event ofoverhydrogenation, i.e. the hydrogenation of monounsaturated compounds,and also isomerization of terminal double bonds to internal doublebonds. At the same time, the concentrations of multiply unsaturatedcompounds have to be reduced to values of usually <10 in the case ofstreams having high contents of multiply unsaturated compounds and to<10 ppm in the fine purification of streams which already have a lowcontent of multiply unsaturated compounds.

Processes and catalysts for the selective hydrogenation of 1,3-butadienein high concentration (˜30-50%) in C₄ streams are described in EP 0 523482, DE 31 19 850, EP 0 992 284 and EP 780 155.

C₄ streams can contain branched multiply unsaturated compounds havingmore than 4 carbon atoms, e.g. isoprene, in addition to unbranchedmultiply unsaturated compounds such as 1,3-butadiene. The presence ofbranched multiply unsaturated compounds is undesirable in the work-up ofC₄ streams or interferes in this work-up for a number of reasons:

-   -   a) If branched multiply unsaturated compounds (e.g. isoprene)        are present in the feed to the MTBE synthesis, these react with        methanol to form the corresponding methyl ethers (e.g.        3-methoxy-3-methylbut-1-ene). If the MTBE is subsequently        cleaved again, the branched multiply unsaturated compounds can        reappear in the product and endanger its purity.    -   b) If the branched multiply unsaturated compounds get into the        feed to the oligomerization of the n-butenes, they deactivate        the oligomerization catalyst.

The selective hydrogenation of both the unbranched and branched multiplyunsaturated compounds could overcome these problems in the work-up of C₄streams. Thus, in the presence of unbranched and branched multiplyunsaturated hydrocarbons, a process for the selective hydrogenation ofunbranched multiply unsaturated C₄-hydrocarbons and a selectivehydrogenation process for the hydrogenation of branched multiplyunsaturated C₅-hydrocarbons would follow one another in order to reducethe concentration of both the branched and unbranched multiplyunsaturated C₄- and C₅-hydrocarbons to values of <10 ppm.

EP 0 081 041 describes a process for the selective hydrogenation ofmultiply unsaturated or acetylenic compounds in low concentrations(<21%, preferably <1%) in C₄ streams. The process is carried out in apurely liquid phase and palladium on an inert support, e.g. aluminiumoxide, serves as catalyst. To suppress the unwanted hydrogenation ofmonounsaturated compounds, carbon monoxide is added in an amount of from0.05 to 20 ppm.

A process for the selective hydrogenation of branched multiplyunsaturated C₅-hydrocarbons in C₅-hydrocarbon mixtures is described inEP 0 556 025.

In Alves et al., Chem. Eng. J. 2004, 99, 45, it is stated that1,3-butadiene can be hydrogenated in a C₄ stream in the presence ofisoprene to concentration values of <10 ppm, but the isopreneconcentration is not reduced to similarly low values.

A person skilled in the art would therefore not expect unbranchedmultiply unsaturated C₄-hydrocarbons and branched multiply unsaturatedC₅-hydrocarbons to be removed together to concentrations of <10 ppm inone process without a significant loss of monounsaturated hydrocarbonsoccurring or significant isomerization of α-olefins to internal olefinstaking place because of the different reaction rates and the differentadsorption constants of these groups of hydrocarbons (cf. Alves et al.,Chem. Eng. J. 2004, 99, 45).

It was therefore an object of the invention to develop a process for theselective hydrogenation of unbranched multiply unsaturatedC₄-hydrocarbons in low concentration in C₄-hydrocarbon mixtures, whichlikewise hydrogenates branched multiply unsaturated C₅-hydrocarbonspresent in the feed stream without the abovementioned undesirablesecondary reactions occurring.

Contrary to the expectations of a person skilled in the art, it has beenshown according to the present invention that unbranched multiplyunsaturated C₄-hydrocarbons and branched multiply unsaturatedC₅-hydrocarbons which occur as constituents in a C₄-hydrocarbon streamcan be hydrogenated to concentration values of <10 ppm in one process.Here, the undesirable hydrogenation of the monounsaturated butenes whichare likewise present in the feed stream and the isomerization of1-butene occur to only a very minor extent.

The present invention accordingly provides a process for the selectivehydrogenation of unbranched, multiply unsaturated C₄-hydrocarbons andbranched, multiply unsaturated C₅-hydrocarbons in hydrocarbon mixtureswith addition of hydrogen and carbon monoxide and using heterogeneoushydrogenation catalysts in a hydrogenation reactor, wherein the ratio ofthe volume of the feed stream into the hydrogenation reactor to thevolume of the hydrogenation catalyst per hour of residence time is notmore than 30 l/lh.

For the purposes of the present invention, unbranched, multiplyunsaturated C₄-hydrocarbons are, in particular, 1,3-butadiene,but-3-en-1-yne and 1,2-butadiene.

For the purposes of the present invention, branched, multiplyunsaturated C₅-hydrocarbons are, in particular, isoprene,2-methylbut-1-en-3-yne, 2-methylbuta-1,2-diene, pent-4-en-2-yne and3-methylbut-3-en-1-yne.

The ratio of the volume of the feed stream into the hydrogenationreactor to the volume of the hydrogenation catalyst per hour ofresidence time [V_(feed)/(V_(cat)*RET), where RET=residence time] isimportant to the process of the invention and the success of theselective hydrogenation. This parameter is known to those skilled in theart as LHSV (liquid hourly space velocity). The volumes mentioned are inliters.

For the purposes of the present invention, the term “feed stream” refersto the totality of all liquid or gaseous reaction components which arefed into the hydrogenation reactor. These are, in particular, thehydrocarbon mixtures in which, inter alia, the unbranched, multiplyunsaturated C₄-hydrocarbons and branched, multiply unsaturatedC₅-hydrocarbons are present but also hydrogen and carbon monoxide.

According to the present invention, the LHSV to be adhered to is notmore than 30 l/lh, in particular from 10 l/lh to 25 l/lh.

Only when the precise limits to the LHSV are adhered to can unbranchedmultiply unsaturated C₄-hydrocarbons and branched multiply unsaturatedC₅-hydrocarbons which occur as constituents in a C₄-hydrocarbon streambe hydrogenated to concentration values of <10 ppm in one processwithout appreciable hydrogenation of the monounsaturated butenes whichare likewise present in the feed stream and isomerization of 1-buteneoccurring. The proportion of unbranched multiply unsaturatedC₄-hydrocarbons and branched multiply unsaturated C₅-hydrocarbons is, inparticular, reduced to less than 10 ppm by the process of the invention.

The process of the invention is operated as a liquid-phase process, i.e.the reaction components are present in the liquid phase over thecatalyst or are introduced in the liquid phase into the hydrogenationreactor.

The addition of hydrogen to the hydrocarbon mixture to be hydrogenatedis thus effected in finely divided form and in such amounts that ahomogeneous liquid phase is always present before entry into thehydrogenation reactor. The stoichiometric ratio (molar ratio) ofhydrogen to the hydrocarbons to be hydrogenated is in the range from 2to 1. The ratio is preferably in the range from 1.5 to 1. It isparticularly preferably in the range from 1.2 to 1.

Carbon monoxide is additionally added to the hydrocarbon mixture to behydrogenated. The content of carbon monoxide in the feed stream is inthe range from 0.05 to 20 ppm of carbon monoxide, based on the mass ofthe hydrocarbon mixture. Preference is given to adding from 0.5 to 5 ppmof carbon monoxide. Amounts above 20 ppm no longer improve thehydrogenation results.

Heterogeneous hydrogenation catalysts are used as catalysts in theprocess of the invention. In particular, the hydrogenation catalysts arecatalysts based on palladium, but the process of the invention is nottied to any particular palladium catalyst. The palladium is preferablypresent in supported form on an inert support material. The supportmaterial is, for example, aluminium oxide, silica gel or activatedcarbon. Preference is given to using aluminium oxide as supportmaterial. The catalyst has a palladium concentration which is in therange from 0.01 to 3%, based on the mass of the support. It ispreferably in the range from 0.1 to 1%, very particularly preferably inthe range from 0.3 to 0.5%. The catalyst has a BET surface area(determined by gas adsorption in accordance with DIN ISO 9277) of from50 to 400 m²/g, preferably from 100 to 300 m²/g, particularly preferablyfrom 200 to 300 m²/g.

The temperature at which the feed stream enters the hydrogenationreactor is usually in the range from 0 to 100° C., preferably in therange from 20 to 80° C., particularly preferably in the range from 30 to60° C. The pressure is usually in the range from 2 to 50 bar, preferablyin the range from 6 to 30 bar, particularly preferably in the range from10 to 25 bar.

The hydrogenation can be carried out in one or more reaction stages. Ifthe amount of multiply unsaturated hydrocarbons present in the feedstream is so large that the necessary stoichiometric amount of hydrogenis no longer soluble in the feed stream, the feed stream can be dilutedby means of the recycle mode. The hydrocarbon mixtures to behydrogenated can contain up to 20% of multiply unsaturated hydrocarbons.

Even without further explanations, it is assumed that a person skilledin the art can utilize the above description in its widest scope. Thepreferred embodiments and examples are therefore merely to beinterpreted as a descriptive disclosure which is not limiting in anyway.

The present invention is illustrated below with the aid of examples.

Alternative embodiments of the present invention can be derived in ananalogous way.

EXAMPLES

The hydrogenation is carried out in a fixed-bed reactor provided with aheating jacket through which a heat transfer oil (Marlotherm SH fromSasol Olefins & Surfactants GmbH) flows. 0.54 liter of a coated catalystcontaining 0.5% of palladium on γ-aluminium oxide in extrudate form isused as catalyst. The specific internal surface area of the catalyst isabout 250 m²/g and the pore volume is about 0.8 cm³/g. The thickness ofthe palladium layer is about 0.05 mm. To produce the hydrocarbon mixtureto be hydrogenated, raffinate 1, 1,3-butadiene and isoprene are mixed.Starting mixture and product mixture are analysed by gas chromatography.

Example 1 According to the Invention

1,3- 1- Isobutane + 2- C5- n- Component Butadiene Isoprene Butenebalance Butene monoenes butane Feed 0.2083 0.2451 28.8933 47.903213.4133 0 9.3368 [% by wt] Output 0.0006 0.0009 28.3557 47.7165 14.08900.2310 9.6063 [% by wt]

Reaction conditions Conversion T P LHSV/ Ratio of CO concentration of [°C.] [bar] l/(l * h) n(H₂)/n(diene) [ppm] 1-butene 30 20 15 1.1 1.3 1.8%

Example 2 Comparative Example

1,3- 1- Isobutene + 2- C5- n- Component Butadiene Isoprene Butenebalance Butene monoenes butane Feed 0.2403 0.2374 29.5211 47.761912.9960 0 9.2433 [% by wt] Output 0.0004 0.0220 29.3735 47.7233 13.19490.2171 9.4688 [% by wt]

Reaction conditions CO Conversion T P LHSV/ Ratio of concentration of [°C.] [bar] l/(l * h) n(H₂)/n(diene) [ppm] 1-butene 30 20 36 1.1 1.0 0.5%

Example 3 According to the Invention

1,3- 1- Isobutane + 2- C5- n- Component Butadiene Isoprene Butenebalance Butene monoenes butane Feed 0.1936 0.2421 29.5211 43.256913.1741 0 13.6122 [% by wt] Output 0.0006 0.0010 29.2902 43.2533 13.59400.2408 13.6201 [% by wt]

Reaction conditions Conversion T P LHSV/ Ratio of CO concentration of [°C.] [bar] l/(l * h) n(H₂)/n(diene) [ppm] 1-butene 40 20 16 1.1 1.3 0.78%

Example 4 According to the Invention

1,3- 1- Isobutene + 2- C5- n- Component Butadiene Isoprene Butenebalance Butene monoenes butane Feed 0.0956 0.2419 28.5082 43.997312.0909 0 15.0648 [% by wt] Output 0.0006 0.0010 27.9097 43.9966 12.76720.2557 15.0692 [% by wt]

Reaction conditions Conversion T P LHSV/ Ratio of CO concentration of [°C.] [bar] l/(l * h) n(H₂)/n(diene) [ppm] 1-butene 40 20 16 1.5 1.3 2.1%

The tables in the examples in each case show the composition of the feedstream and of the output stream of the fixed-bed reactor under variousreaction conditions. Example 1 shows the results of the hydrogenation ofabout 2000 ppm of 1,3-butadiene and about 2400 ppm of isoprene at anLHSV according to the invention. It can be seen that both 1,3-butadieneand isoprene can be hydrogenated to a proportion by mass of less than 10ppm without large proportions of the products of value 1-butene and2-butene being lost. 1-Butene is converted to an extent of only 1.8%(conversion=(m_(in)−m_(out))/m_(in)).

In Example 2, an LHSV of 36 l/(l*h) analogous to EP 0 081 041 is set.Here too, about 2000 ppm of 1,3-butadiene and about 2400 ppm of isopreneare present in the feed stream. However, at this high LHSV, theproportion by mass of isoprene is reduced only to a value of about 200ppm, which is not acceptable in the fine purification of C₄ fractions.

In Example 3, the temperature is increased to 40° C. Here too, about2000 ppm of 1,3-butadiene and about 2300 ppm of isoprene can behydrogenated to a proportion by mass of less than 10 ppm without largeproportions of the products of value being lost. 1-Butene is convertedto an extent of 0.78%, while 2-butene again displays a negativeconversion. The proportion of butanes as an indication of totalhydrogenation likewise increases only by a value of less than 100 ppm.

In Example 4, the concentration of 1,3-butadiene is reduced to about1000 ppm and at the same time the ratio of hydrogen to diene isincreased from 1.1 to 1.5. Here too, about 2000 ppm of 1,3-butadiene andabout 2300 ppm of isoprene can be hydrogenated to a proportion by massof less than 10 ppm without large proportions of the products of valuebeing lost. As a result of the increased hydrogen/diene ratio, 2.1% of1-butene are now converted, but this is still a very low value. Theconversion of 2-butene at the same time becomes more negative, whichindicates increased isomerization of 1-butene to 2-butene. However,total hydrogenation to butanes takes place to only an insignificantextent.

The invention claimed is:
 1. A process, comprising: selectivelyhydrogenating an unbranched, multiply unsaturated C₄-hydrocarbon and abranched, multiply unsaturated C₅-hydrocarbon in a hydrocarbon mixturewith addition of hydrogen and carbon monoxide in the presence of aheterogeneous hydrogenation catalyst in a hydrogenation reactor, whereina ratio of the volume of a feed stream comprising the hydrocarbonmixture, hydrogen, and carbon monoxide into the hydrogenation reactor tothe volume of the hydrogenation catalyst per hour of residence time isfrom 10 to 25 l/h, wherein a temperature at which the feed stream entersthe hydrogenation reactor is from 30 to 60° C., and wherein a content ofcarbon monoxide in the feed stream is from 0.5 to 5 ppm, based on themass of the hydrocarbon mixture.
 2. The process of claim 1, wherein theheterogeneous hydrogenation catalyst is a palladium catalyst.
 3. Theprocess of claim 1, being a liquid-phase process.
 4. The process ofclaim 2, wherein the palladium catalyst is a supported catalystcomprising palladium and a support material.
 5. The process of claim 4,wherein the support material is aluminum oxide, silica gel, or activatedcarbon.
 6. The process of claim 5, wherein a content of the palladiumcatalyst is from 0.01 to 3%, based on a mass of the support material. 7.The process of claim 5, wherein a content of the palladium catalyst isfrom 0.1 to 1%, based on a mass of the support material.
 8. The processof claim 5, wherein a content of the palladium catalyst is from 0.3 to0.5%, based on a mass of the support material.
 9. The process of claim2, wherein the palladium catalyst has a BET surface area of from 50 to400 m²/g, determined by gas adsorption in accordance with DIN ISO 9277.10. The process of claim 2, wherein the palladium catalyst has a BETsurface area of from 100 to 300 m²/g, determined by gas adsorption inaccordance with DIN ISO
 9277. 11. The process of claim 2, wherein thepalladium catalyst has a BET surface area of from 200 to 300 m²/g,determined by gas adsorption in accordance with DIN ISO 9277.